A need has arisen for the payloads of spacecraft such as communications satellites to have greater signal traffic flexibility. This has been brought about by the increasing requirement for large numbers of coverage areas and hence antenna beams. This trend can be accounted for as follows:
1. Conventionally, satellites have generated large coverage areas on the Earth's surface using relatively small antennas on the satellite, due to limitations in the satellite technology. In order to increase the satellite's effective output power (EIRP--Effective Isotropic Radiated Power) and input sensitivity (G/T--Gain to noise temperature ratio) the satellite antenna gain can be increased resulting in a smaller beam. Many such beams are then used to provide the same total coverage of the Earth's surface as the original large beam. FIGS. 1(a) and (b) show examples of this conventional technique of multiple beam coverage.
2. The first generations of operational communications satellites were designed and used for international communications and essentially required almost global coverage. More recently, communications satellites have been employed by individual countries, or relatively small groups of countries, to provide domestic or intra-regional communications. Smaller coverage areas are needed for these applications.
However, the financial viability of domestic communications satellites is being called into question and there is now a tendency to `pool` the resources of a communications satellite between several countries, with the ability to `reconfigure` the resources within the satellite to serve one or other of the countries at different times during the satellite's lifetime. This gives rise to the need for many separate antenna beams, with an uncertain allocation of satellite resources (bandwidth and power) between those beams.
Another trend, linked to the above use of many smaller antenna beams, is for communications satellites to be larger and to carry a large number of transponders (up to fifty). This results in the need to be able flexibly to connect (switch, etc) up to fifty transponders between, say, twenty to forty antenna beams.
Satellite lifetimes have evolved over the last twenty years from two to three years to ten to fifteen years. The ability to accurately predict the exact traffic requirements this far in advance (plus the three year satellite construction period) is becoming a major problem.
Two further relevant factors are:
1. The trend towards using small Earth stations is the result of trading-off the relative costs of the satellite and the Earth stations in a total system. To make the Earth station smaller and less expensive and so to allow such Earth stations to proliferate, involves less efficient use of the satellite resource. In particular, this means using more satellite power per link than would be necessary for operation with a larger Earth station. This inevitably means that the satellite `transponder` is operating in a `power limited` mode and actually has more bandwidth available than it can usefully use.
2. Small and cheap Earth stations also encourage the use of SCPC (Single Channel Per Carrier) mode of operation which means that there are many carriers simultaneously present within the same satellite transponder bandwidth. Each carrier will support its group of Earth stations. To avoid intermodulation distortion dominating the performance of the links, the satellite HPA (High Power Amplifier) has to be operated in a linear mode, commonly called `backed-off`.
It has been proposed to meet these requirements and trends by providing conventional switching networks in a communications satellite between the high power amplifiers (HPA's) and the transmit antenna beam ports but this will severely limit the satellite operating capability and signal traffic flexibility by the large mass, great size, great complexity and high power loss of such conventional switching networks.